PALEOZOIC GROUP. 307 



IRRUPTIONS DURING AND SUCCEEDING THE SILURIAN PERIOD. 



The problems connected with the dynamic history of the Silurian are not 

 as difficult to solve as some of the earlier complications, and yet there are a 

 few points which still remain doubtful, on account of more recent uplifts and 

 because of unfilled gaps in the record. There are several localities in which 

 the Silurian system presents structural features very difficult to explain upon 

 any other hypothesis than that of upthrow or downthrow of the strata along 

 the courses of the Post-Texan and Post-Fernandan orographic movements — 

 the north and northwest trends, respectively. The lower portion of Katemcy 

 Creek, in McCulloch County, the headwaters of Cold and Little Llano creeks, 

 in Llano County, and the Colorado River along much of the eastern bound- 

 ary of Llano County, are all bordered upon the east by abrupt cliffs of Silu- 

 rian strata which extend southward as promontories, or bay like prolongations, 

 sharply outlined upon the west by meridianal fault lines. In the southwest, 

 along the upper portion of James River, in Mason County, and in the north- 

 west, along the San Saba River, as well as over intermediate territory on Bluff 

 and Little Bluff creeks and elsewhere, a similar manifestation of the north- 

 west trend is apparent in Silurian strata. It would seem that these move- 

 ments ceased before the deposition of the San Saba series, and it may be 

 that they can all be referred to slips along the earlier uplifts, although some 

 of the facts seem to require the solution suggested above. 



But there is another series of breaks which certainly represent a subsequent 

 dynamic event. These trend very uniformly in the course north 25 degrees 

 east,* and so far as my observations go they are not characteristic of any 

 terranes of later age than the Silurian. 



Invariably this trend has broken all the others which have been regarded 

 as earlier in this report, and they are always broken by those to be herein- 

 after classed as later trends. Within the area now covered by Silurian strata 

 the results of this Post- Silurian uplift are manifested by faults and changes 

 of dip, but very seldom has erosion brought to view the igneous agents. It 

 is probable that a compaiison of such exposures (of which none are now cer- 

 tainly known) with outcrops of the same magma in the much denuded inner 

 area will give us the means of determining conclusively whether the Silurian 

 strata ever extended over the latter district. At present the evidence, such 

 as it is, favors a contrary verdict. There are two belts in which the peculiar 

 phenomena of the Post- Silurian uplift are plainly exhibited over limited 

 areas. One of these is in the north 25 degrees east trend, north of Packsad- 

 dle Mountain, on Honey Creek and the Llano River in Llano County; the 



*North 16 degrees east, magnetic; variation, 9 degrees 15 minutes east; corrected, north 

 25 degrees 15 minutes east. 



